1. Field of the Invention
The present invention relates generally to a millimeter-wave band frequency optical oscillator that can be used as an oscillation frequency signal source for a millimeter-wave forwarded to wireless subscribers from a base station of a millimeter-wave wireless subscriber communication system for next generation (e.g., fifth generation) ultra-high speed wireless internet service. More particularly, the present invention relates to a modulation frequency tunable optical oscillator including an optical fiber amplifier and an optical fiber grating mirror which are connected to each of input/output ports of a loop mirror to realize simultaneous oscillation in two laser modes suitable for each wavelength.
2. Discussion of Related Art
Generally, a communication technology that uses a millimeter-wave band to provide high-capacity information of about 100 Mbps to subscribers is being studied and developed as a next generation (e.g., fifth generation) personal wireless technology by institutes at home and abroad studying communication technologies. Because the millimeter-wave band is greatly attenuated on air, wireless communication must be made in a local area having an effective distance less than 200 m. It, therefore, needs a repeater at a position close to subscribers, wherein a high frequency optical signal in a millimeter-wave band, which carries information, is forwarded to the repeater through an optical line. At this time, a high frequency optical oscillator is used as an oscillation frequency signal source that generates the high frequency optical signal.
A method of using semiconductor high frequency optical modulation and a method of using self-modulation in a resonator have been studied and developed in the art from five to six years ago. The method of using optical modulation is being studied and developed with a frequency region having the highest frequency of 40 GHz. As the method using self-modulation, optical fiber oscillators having a complex resonator structure have been developed with a 60˜80 GHz frequency region.
For example, a ring resonator having an optical fiber grating mirror was proposed in Korean Patent Application No. 2002-3529 (Jan. 22, 2002). A similar high frequency laser light source based on the ring resonator was also developed.
However, in the conventional optical oscillator as described above, a laser mode reciprocating along a pair of optical fiber grating mirrors passes over about two times the resonance length compared to a transmitting laser mode. Accordingly, there are great differences in a birefringence phenomenon and a resonant frequency between the two modes. It increases a modulation frequency between the two modes, but relatively reduces an adjustment width of a polarization adjuster (polarization state) to make simultaneous oscillation in the two modes.